A large portion of the metabolic energy available to cells is spent on concentrating nutrient molecules within the cell and on maintenance of appropriate intracellular and extracellular electrolyte concentrations. The proposed research is aimed at increasing the knowledge about the mechanisms, cell biology and molecular basis of sugar, amino acid, and electrolyte transport across the plasma membrane of animal cells. To characterize the various transport systems at the molecular level, flux measurements of the appropriate solutes will be carried out in vitro across the membrane of isolated plasma membrane vesicles. The fluxes will be determined under experimental conditions which allow inferences to be made about the properties of the membrane with respect to the overall translocation mechanism catalyzed, as well as to the dynamic properties of the transport systems (dependence on substrates concentration, electrical potential, temperature, ionic environment, etc.) Mathematical tools are to be developed to deal with information from flux measurements in heterogeneous populations of membrane vesicles. The particular plasma membranes whose transport systems are to be investigated are derived from the luminal and contraluminal pole of small intestinal epithelial cells. The research is thought to be of general importance for the plasma membrane aspects of cell biology and metabolism and of particular importance for molecular aspects of small intestinal physiology and pathophysiology.